This invention relates to a fuel assembly for a nuclear reactor and more particularly to fuel assembly for a boiling water reactor capable of ensuring a long operation cycle and maintaining a high shut-down margin.
A fuel assembly for a boiling water reactor (BWR) is constructed by a square channel box in which a number of fuel rods each comprising a metal cladding tube in which neclear fuel material is packed, are regularly arranged. The reactor core of the BWR includes a plurality of cells each comprising a cruciform control blade and four fuel assemblies surrounding the control blade and these cells are arranged in a regulated manner in the core. Namely, each fuel assembly and control blade have axes perpendicular and parallel to each other, and a coolant functioning as moderator flows from the lower portion towards the upper portion of the reactor core.
Concerning the BWR, steam voids are not formed in the portion near the lower end of the active core region, i.e., the lower end of a heat generating portion at which an exothermic reaction is performed, but many voids are generated at the area above the central portion of the reactor core, and the generated voids move up towards the upper portion of the reactor core. Accordingly, the void fraction in the BWR becomes high towards the upper portion of the reactor core, and as a result, the moderation characteristics of neutrons are lowered and hence the output power is also lowered. In order to obviate these defects, in a conventional technique, increasing the enrichment of the fissile nuclides to be contained in the fuel at portions with a high void fraction has been performed, and mixing a burnable poison with the fuel element to suppress the increasing in the power output at a portion of low void fraction has also been performed.
For the reasons described above, in the BWR, the burn-up at the upper portion of the core is liable to be delayed, and hence the concentration of U-235 becomes relatively higher than that of other portions of the core. In addition, since a fissile nuclide such as Pu-239 is produced relatively in higher rate at the upper portion of the core due to neutron-spectral hardening through higher void fraction compared with those at the lower portion, it is difficult to maintain the shut-down margin of the reactor core at the upper portion thereof. Moreover, recently, many efforts have been made for elongating the operation cycle of the reactor and for extending the brun-up of the fuel in order to satisfy the economic requirements. In these cases, however, the enrichment of the fuel is necessarily increased, so that the subcriticality at the upper portion of the reactor core is further reduced and the maintenance of the shut-down margin of the reactor is made more difficult. These technical defects resulted in difficulty of ensuring a long operation cycle for nuclear reactor cores of the conventional type. Moreover, to make the subcriticality during reactor shut-down period larger to ensure the maintenance of the reactor shut-down margin, it is effective to minimize the difference obtained by subtracting the infinite multiplication factor during the power operation period from that during the cold operation period, i.e., the difference of the reactivities at these operation periods. For the reason described above, there is provided a method for improving the reactor shut-down margin by reducing the enrichment of the fuel rods in the first layer of the fuel assembly (i.e., fuel rods arranged along the inner wall of the channel box), for example, as disclosed in Japanese Patent Publication No. 49946/1987.
However, with the conventional method described above, no consideration is paid for the worth of the control rods or blades to be inserted into the reactor core at the reactor shut-down time, and hence, the method is not sufficient for the improvement of the reactor shut-down margin.